This invention relates generally to the suppression of arcs across electrical power contacts, by the activation of a controlled electronic device which is coupled across the contacts, each time the power contacts are either opened or closed. The invention is disclosed particularly in relation to an electrical power contactor having a movable plunger and at least one pair of power contacts. In the disclosed form of the invention, a gate controlled thyristor is coupled across each pair of power contacts. A switch is connected to the contactor plunger and is used to activate a thyristor gating circuit for an interval of time.
In the use of electrical power switching devices, the opening and closing of the switch contacts can result in arcing across the contacts. Such arcing is objectionable because, for example, it produces radio frequency interference in other electrical equipment in the vicinity of the switching device. Equally important, such arcing degrades the switch contacts and reduces their useful life.
One technique which has been employed to prevent arcing across power contacts is to couple a thyristor, such as a triac, across the contacts. The triac is then gated into conduction prior to each opening or closing of the power contacts. Such power contacts to be protected from arcing are typically embodied in a power switching device, which may be, for example, a power relay, a motor starter, or a contactor. In such devices, typically a movable contact is moved into engagement with a pair of spaced power contacts by an actuator or plunger movable under the influence of a solenoid coil.
Therefore, to close the pair of power contacts, for example, a control voltage is applied to the solenoid coil, moving the plunger and the movable contact toward, and into engagement with, the pair of fixed contacts. Due to the inertia of the plunger, there is a delay between the application of the control voltage and the movement of the plunger. There is a further delay in the closure of the power contacts due to the distance of travel of the plunger before the movable contact reaches the fixed contacts.
In the past, gate controlled thyristors coupled across the power contacts have been gated into conduction prior to opening or closing of the power contacts in various ways. Some of these gating techniques take advantage of the above-mentioned inherent delay characteristics of the power switching devices. For example, a portion of the control current supplied to the solenoid of the switching device may be used to gate an arc suppression thyristor. Or the movement of the switching device plunger may be used to close an auxiliary switch contact, supplying a gating signal to the thyristor. In either of these systems, the thyristors would normally remain conductive while the power contacts are closed.
It has also been found, however, that it is preferable to remove the gating signal from the thyristor, even after the closing of the power contacts. The primary reason for this is that the power contacts may develop a significant resistance therebetween, requiring the thyristor to carry a substantial continuous current, which can overload the thyristor.
Preferred thyristor gating techniques, therefore, provide for gating on the thyristor before the power contacts either open or close, maintaining the thyristor conductive until after the contacts have opened or closed, and thereafter removing the gating signal from the thyristor, turning off the thyristor. This has been accomplished in a number of ways.
In one approach, an auxiliary contact is mechanically coupled to the switching device plunger. In this approach, the auxiliary contact momentarily closes an auxiliary switch as the plunger moves through its distance of travel to open or close the power contacts. While this auxiliary switch is momentarily closed, the thyristor gate is coupled to a power source, and the thyristor is gated on. When the auxiliary switch opens, the gating signal is removed and the thyristor is turned off. This thyristor gating technique requires a custom designed switching device susceptible of accurate calibration since the timing of the actuation of the thyristor depends upon the timing of both the opening and the closing of the auxiliary switch.
In another approach, a secondary coil is linked to the solenoid control coil and connected to the gate of the triac. This requires the use of a dc supply to energize the solenoid coil so that the thyristor gate receives a control signal only when the power contacts are opened or closed. Such a circuit requires that a dc supply is available to energize the solenoid, as well as the use of a switching device having a solenoid suitably actuable by a dc supply.
In other approaches to gating an arc suppression thyristor, the solenoid coil of the switching device and a gating circuit for the thyristor are both controlled by an additional arc suppression control circuit. In this case, the control voltage for the solenoid of the power switching device is not coupled directly to the solenoid, but is instead received by the arc suppression control circuit. The arc suppression control circuit contains a timing circuit to provide a thyristor gating signal for the desired interval of time to obtain satisfactory arc suppression. This timing circuit may take the form, for example, of a digital timer or a capacitor discharge timing network. Such additional control circuit arrangements permit relatively accurate electronic timing of thyristor gating, but at the cost of the introduction of fairly elaborate circuitry interposed between the externally applied solenoid control signal and the power switching device itself.
All of the foregoing arc suppression arrangements for power switching devices, therefore, require either custom design and accurate calibration of a mechanical switching device or the interposition of expensive additional control circuitry between the externally applied control signal and the power switching device. As a result, each of the prior art arc suppresion arrangements was subject to one or more serious disadvantages. The mechanical switching arrangements were delicate and difficult to calibrate; the all electrical arrangements were relatively expensive. Moreover, many of these prior art arrangements were of larger size, requiring excessive space in the control cabinet.